With emergence of digital camera technology, cameras have been popular to utilise for a plurality of purposes. Today cameras are used widely not only for photography but also for new purposes. Surveillance of public areas or private property has become popular to utilise cameras for. Digital camera technology is also applied for monitoring, navigation, etc. Computer vision is a technical field where a stereo camera arrangement is utilised for imaging a scene.
In stereo camera arrangements two cameras are used to produce a 3D (Three dimensional) image or model of an environment or scene. In a stereo camera arrangement two cameras are rectified towards a scene from two different directions and there is a distance between the cameras. By rectifying the cameras with a plurality of angles between them, information of a distance to an object could be determined. To simulate the human vision, a suitable distance between the cameras is between 5 and 10 cm which correspond to the distance between the left and the right eye of a person.
The cameras of a stereo camera arrangement will be calibrated to operate properly. Intrinsic parameters of the cameras will be adjusted when calibrating the cameras, where typical intrinsic parameters to adjust are direction, focus, etc.
Traditionally, intrinsic parameters of cameras for stereo purposes have been set when manufacturing stereo camera arrangements. Alternatively, the intrinsic parameters may be manually set in conjunction with installation of the cameras, and manual adjustments thereof will be performed when needed.
However, due to the fact that the amount of stereo camera arrangements increases, there are a large number of cameras to adjust intrinsic parameters for. In addition, new applications have been developed and stereo camera arrangements have been popular to use also in environments where the physical conditions require regular adjustments of intrinsic parameters. For instance, stereo camera arrangements in vehicles will typically be affected by shaking and may need to be adjusted during transport to operate properly.
With reference to the FIGS. 1a-b, which are schematic illustration views, a stereo camera arrangement will now be described in accordance with one example. In FIG. 1a the stereo camera arrangement is illustrated in a perspective view, and in FIG. 1b, the same stereo camera arrangement is illustrated from above.
The stereo camera arrangement 100 comprises a first camera 102 and a second camera 104. Both the first camera 102 and the second camera 104 are respectively connected to a control unit 106 (not shown in FIG. 1b). The cameras 102, 104 are arranged to capture respective images of a scene 108. The control unit 106 is arranged to calibrate the cameras 102, 104.
Today, some methods are applied for calibration of stereo cameras. These known methods are quite complex and require a substantial amount of calculation capacity or additional references. For instance, Nedevschi et al. (On-Line Calibration Method for Stereovision Systems Used in Vehicle Applications) present a method where markers on a road are captured by two cameras. Hansen et al. (Online Continuous Stereo Extrinsic Parameter Estimation) present a method where a baseline is determined.
Thus, there is a problem to devise a cost-effective and flexible method for appropriate calibration of cameras in stereo camera arrangements.